As those skilled in the art will understand, induction heating is the heating of a nominally electrically conducting material by eddy currents induced by a time-varying magnetic field generated by an induction coil. The principle of induction heating is similar to that of a transformer. The induction coil can be considered the primary winding of a transformer, with the workpiece to be heated as a single-turn secondary. When an alternating current flows in the induction coil, secondary currents will be induced in the workpiece. These induced currents are called eddy currents, and the power dissipated by the eddy currents as they flow in the workpiece heats the workpiece.
Induction heating is widely employed in industry for a variety of industrial processes. While carbon steel is the most common material heated, induction heating is also used with many other conducting materials such as stainless steel, aluminum, brass, copper, nickel, and titanium products. Induction heating is widely employed in the melting, holding and superheating of ferrous and nonferrous metals in coreless and channel furnaces; in the forging, forming, and rolling of slabs, billets and bars; in heat treatment, such as hardening, annealing, and tempering applications; surface conditioning, such as curing of coatings, sintering, and semiconductor processing; and metal joining, such as welding, brazing, and soldering.
Induction heating offers several advantages over conventional (i.e., fossil fuel) heating processes. Heating is induced directly into the material. It is therefore an extremely rapid method of heating. It is not limited by the relatively slow rate of heat diffusion in conventional processes using surface-contact or radiant heating methods. Because of skin effect, the heating is localized and the heated area is easily controlled by the shape and size of the induction coil. Induction heating is easily controllable, resulting in uniform high quality of the product. Induction heating lends itself to automation, in-line processing, and automatic-process cycle control. Startup time is short, and standby losses are low or nonexistent. Working conditions are better because of the absence of noise, fumes, and radiated heat.
For efficient heating, the frequency of the alternating current in the induction coil, and thus the frequency of the magnetic field, must be high enough so that the depth of current penetration is less than one-third the diameter or cross-section of the material being heated. When the workpieces are small, it is necessary to use higher frequencies to efficiently heat the workpiece. Likewise, higher frequencies must be used when it is necessary to concentrate the heat near the surface, as in surface hardening operations. Solid-state power supplies are most often used to generate the high frequency power needed to excite the induction coil.
Various forms of solid-state power supplies exist for supplying power to induction coils for induction heating. One example is a self-excited class C oscillator using a single bipolar transistor, which generates power outputs of approximately 100 W into the megahertz frequency range. However, even small induction heating devices require power levels of 200 W to 1000 W of output.
U.S. Pat. No. 4,001,725, by the same inventor as the present invention, discloses a solid-state self-excited oscillator employing several series-connected bipolar transistors to provide a high power output. The circuit disclosed in U.S. Pat. No. 4,001,725 works well at power levels up to about 1 kW. However, the low rated collector-to-emitter voltages of available bipolar transistors limits power output at the 100 kHz - 200 kHz frequency range. In addition, applying series-connected bipolar transistors can introduce unequal collector-to-emitter RF voltage and power dissipation among the transistors. A transistor breakdown could cause failures of the remaining transistors by excessive voltage. Also, secondary breakdown in bipolar transistors may be difficult to control in circuits where there are wide variations in operating conditions, as is the case with induction heating. Moreover, power losses within the bipolar transistors are high when switching high currents at high frequency (100 kHz - 200 kHz).
It is therefore an object of the present invention to provide a practical, solid-state power supply capable of supplying a high frequency, high power output to a load with flexibility to match a wide range of load impedances, which is readily expandable in power output, and which does not exhibit the drawbacks of prior power supplies.